Semiconductor device

ABSTRACT

To provide a highly reliable semiconductor device. To provide a semiconductor device which prevents a defect and achieves miniaturization. An oxide semiconductor layer in which the thickness of a region serving as a source region or a drain region is larger than the thickness of a region serving as a channel formation region is formed in contact with an insulating layer including a trench. In a transistor including the oxide semiconductor layer, variation in threshold voltage, degradation of electric characteristics, and shift to normally on can be suppressed and source resistance or drain resistance can be reduced, so that the transistor can have high reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification relates to a semiconductor device utilizing a semiconductor element and a manufacturing method thereof

2. Description of the Related Art

A technique by which a transistor is formed using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A technique by which a wide-gap semiconductor such as an oxide semiconductor is used for a semiconductor thin film which can be applied to a transistor has attracted attention.

For example, in Patent Document 1, it has been confirmed that an oxide semiconductor containing an In—Ga—Zn-based oxide is applicable to a channel formation region of a thin film transistor.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.     2004-103957

SUMMARY OF THE INVENTION

A transistor used for a semiconductor device is required to have high reliability and reduced deterioration and variation. Thus, by using the highly reliable transistor, a semiconductor device can be highly reliable.

In order to achieve high-speed operation, low power consumption, high integration, cost reduction, or the like of a transistor, it is necessary to miniaturize a transistor.

In the case where a transistor is miniaturized, a problem of a short-channel effect is also caused. The short-channel effect refers to degradation of electric characteristics which becomes obvious with miniaturization of a transistor (a reduction in channel length (L)). The short-channel effect results from the effect of an electric field of a drain on a source. Specific examples of the short-channel effect are a decrease in threshold voltage, an increase in subthreshold swing (S value), an increase in leakage current, and the like.

In the view of the above, it is an object of one embodiment of the disclosed invention to provide a highly reliable semiconductor device. Further, it is another object of one embodiment of the disclosed invention to provide a semiconductor device which prevents defects and achieves miniaturization.

According to one embodiment of the disclosed invention, an oxide semiconductor layer in which the thickness of a region serving as a source region or a drain region is larger than the thickness of a region serving as a channel formation region is formed in contact with an insulating layer including a trench. In a transistor including the oxide semiconductor layer, variation in threshold voltage, degradation of electric characteristics, and shift to normally on can be suppressed and source resistance or drain resistance can be reduced, so that the transistor can have high reliability. Specifically, the following structure can be employed for example.

According to one embodiment of the present invention, a semiconductor device includes an insulating layer including a trench, a wide-gap semiconductor layer provided in contact with an inner wall surface and a bottom surface of the trench and the uppermost surface of the insulating layer, a source electrode and a drain electrode electrically connected to the wide-gap semiconductor layer, a gate insulating layer provided over the wide-gap semiconductor layer, and a gate electrode provided over the gate insulating layer to fill the trench. In the wide-gap semiconductor layer, the thickness of a region in contact with the inner wall surface of the trench and the thickness of a region in contact with the bottom surface of the trench are each smaller than the thickness of a region in contact with the uppermost surface of the insulating layer.

According to another embodiment of the present invention, a semiconductor device includes an insulating layer including a trench, a wide-gap semiconductor layer provided in contact with an inner wall surface and a bottom surface of the trench and the uppermost surface of the insulating layer, a source electrode and a drain electrode electrically connected to the wide-gap semiconductor layer, a gate insulating layer provided over the wide-gap semiconductor layer, and a gate electrode provided over the gate insulating layer to fill the trench. The aspect ratio of the trench is greater than or equal to 0.5 and less than or equal to 5.0. In the wide-gap semiconductor layer, the thickness of a region in contact with the inner wall surface of the trench and the thickness of a region in contact with the bottom surface of the trench are each smaller than the thickness of a region in contact with the uppermost surface of the insulating layer.

In one or both of the above semiconductor devices, the thickness of part of the wide-gap semiconductor layer in contact with the uppermost surface of the insulating layer is preferably greater than or equal to 1.5 times and less than or equal to 10 times as large as each of the thicknesses of parts of the wide-gap semiconductor layer in contact with the inner wall surface and the bottom surface of the trench.

In any one of the above semiconductor devices, an oxide semiconductor layer may be used as the wide-gap semiconductor layer.

Note that in this specification and the like, the term such as “over” does not necessarily mean that a component is placed “directly on” another component. For example, the expression “a gate electrode over a gate insulating layer” can mean the case where there is an additional component between the gate insulating layer and the gate electrode. The same applies to the term “below”.

In addition, in this specification and the like, the term such as “electrode” or “wiring” does not limit a function of a component. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. In addition, the term “electrode” or “wiring” can also mean a combination of a plurality of “electrodes” and “wirings”, for example.

Note that functions of a “source” and a “drain” might interchange when a transistor of opposite polarity is used or the direction of current flow is changed in circuit operation, for example. Thus, in this specification, the terms “source” and “drain” can interchange.

Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected to each other through an “object having any electric action”. Here, there is no particular limitation on an “object having any electric action” as long as electric signals can be transmitted and received between components that are connected to each other.

Examples of an “object having any electric action” include a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions in addition to an electrode and a wiring.

According to one embodiment of the disclosed invention, a method for manufacturing a semiconductor device which prevents a defect and achieves miniaturization can be provided.

In addition, according to one embodiment of the disclosed invention, the size of a transistor can be sufficiently reduced. When the size of the transistor is sufficiently reduced, the size of a semiconductor device is also reduced and thus the number of semiconductor devices manufactured from one substrate is increased. Accordingly, manufacturing cost of the semiconductor device can be reduced. Further, since the semiconductor device is downsized, the semiconductor device which is substantially the same in size with further increased function can be realized. Alternatively, high integration of the semiconductor device can be achieved. Furthermore, effects of high-speed operation, low power consumption, and the like due to a miniaturization of the transistor can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a plan view and a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention;

FIGS. 2A to 2D are cross-sectional views illustrating manufacturing steps of a semiconductor device of one embodiment of the present invention;

FIGS. 3A to 3C are a cross-sectional view, a plan view, and a circuit diagram illustrating a semiconductor device of one embodiment of the present invention;

FIGS. 4A and 4B are a circuit diagram and a perspective view illustrating a semiconductor device of one embodiment of the present invention;

FIGS. 5A and 5B are a cross-sectional view and a plan view illustrating a semiconductor device of one embodiment of the present invention;

FIGS. 6A and 6B are circuit diagrams each illustrating a semiconductor device of one embodiment of the present invention;

FIG. 7 is a block diagram illustrating a semiconductor device of one embodiment of the present invention;

FIG. 8 is a block diagram illustrating a semiconductor device of one embodiment of the present invention;

FIG. 9 is a block diagram illustrating a semiconductor device of one embodiment of the present invention; and

FIG. 10 is a cross-sectional TEM photograph of a sample manufactured in Example.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described below with reference to drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that the modes and details of the present invention can be modified in various ways without departing from the spirit and scope thereof. Therefore, the present invention should not be interpreted as being limited to the description in the following embodiments. Note that in the following embodiments and example, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and explanation thereof will not be repeated.

Note that the position, size, range, or the like of each component illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like as disclosed in the drawings and the like.

Note that in this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components and do not limit the number.

(Embodiment 1)

In this embodiment, an example of a semiconductor device and a manufacturing process thereof according to one embodiment of the disclosed invention will be described with reference to FIGS. 1A and 1B.

In this embodiment, an example in which an oxide semiconductor is used as a wide-gap semiconductor which can be applied to a transistor is described. As an oxide semiconductor, an oxide semiconductor having a band gap larger than at least 1.1 eV, which is the band gap of silicon, can be applied; for example, an In—Ga—Zn—O-based oxide semiconductor having a band gap of 3.15 eV, indium oxide having a band gap of about 3.0 eV, indium tin oxide having a band gap of about 3.0 eV, indium gallium oxide having a band gap of about 3.3 eV, indium zinc oxide having a band gap of about 2.7 eV, tin oxide having a band gap of about 3.3 eV, zinc oxide having a band gap of about 3.37 eV, or the like can be preferably used. Note that the wide-gap semiconductor which can be applied to the semiconductor device of one embodiment of the present invention is not limited to the above oxide semiconductors. Alternatively, gallium nitride, gallium oxynitride, gallium zinc oxynitride, or the like may be used. With the use of such a material, the off-state current of the transistor can be kept extremely low.

FIG. 1A is a plan view of a transistor 162 in this embodiment. FIG. 1B is a cross-sectional view taken along line A-B in FIG. 1A.

The transistor 162 illustrated in FIGS. 1A and 1B includes an insulating layer 130 provided with a trench 131, an oxide semiconductor layer 144 provided in contact with an inner wall surface and a bottom surface of the trench 131 and with the uppermost surface of the insulating layer 130 (a surface parallel to a surface where the insulating layer 130 is formed), a source electrode 142 a and a drain electrode 142 b electrically connected to the oxide semiconductor layer 144, a gate insulating layer 146 provided over the oxide semiconductor layer 144, and a gate electrode 148 provided over the gate insulating layer 146, which fills the trench 131.

In the oxide semiconductor layer 144 provided in the transistor 162 illustrated in FIGS. 1A and 1B, the thickness of a region in contact with the inner wall surface of the trench 131 and the thickness of a region in contact with the bottom surface of the trench 131 are each smaller than the thickness of a region in contact with the uppermost surface of the insulating layer 130 (a region of the insulating layer 130 other than the region where the trench 131 is formed).

The region of the oxide semiconductor layer 144 in contact with the uppermost surface of the insulating layer 130 serves as a source region or a drain region. The thickness of the region is increased, so that source resistance or drain resistance can be reduced. On the other hand, the region of the oxide semiconductor layer 144 in contact with the inner wall surface of the trench 131 and the region of the oxide semiconductor layer 144 in contact with the bottom surface of the trench 131 overlap with the gate electrode 148 and form a channel. When the thicknesses of the regions are decreased, a path through which current flows in the transistor 162 can be reduced; thus, variation in threshold voltage, degradation of electric characteristics, and shift to normally on of the transistor 162 can be suppressed. Thus, the reliability of the transistor including the oxide semiconductor layer 144 with the different thicknesses can be increased.

For example, in the oxide semiconductor layer 144, the thickness of the region in contact with the uppermost surface of the insulating layer 130 is preferably 1.5 times to 10 times, more preferably 2 times to 5 times as large as each of the thicknesses of the region in contact with the inner wall surface of the trench 131 and the region in contact with the bottom surface of the trench 131. When the thickness of the region in contact with the uppermost surface of the insulating layer 130 is 1.5 times or more as large as each of the thicknesses of the region in contact with the inner wall surface of the trench 131 and the region in contact with the bottom surface of the trench 131, the above advantageous effects can be sufficiently obtained. In addition, when the thickness of the region in contact with the uppermost surface of the insulating layer 130 is 10 times or less as large as each of the thicknesses of the region in contact with the inner wall surface of the trench 131 and the region in contact with the bottom surface of the trench 131, a layer provided over and in contact with the oxide semiconductor layer 144 can be formed with good coverage.

The oxide semiconductor layer 144 included in the transistor 162 is preferably a highly purified oxide semiconductor layer by sufficiently removing an impurity such as hydrogen or sufficiently supplying oxygen. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is 5×10¹⁹ atoms/cm³ or lower, preferably 5×10¹⁸ atoms/cm³ or lower, more preferably 5×10¹⁷ atoms/cm³ or lower. Note that the hydrogen concentration of the oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). In the oxide semiconductor layer 144 which is highly purified by sufficiently reducing the hydrogen concentration and in which defect levels in an energy gap due to oxygen deficiency are reduced by supplying a sufficient amount of oxygen, the carrier concentration is lower than 1×10¹²/cm³, preferably lower than 1×10¹¹/cm³, more preferably lower than 1.45×10¹⁰/cm³. For example, the off-state current (per unit channel width (1 μm) here) at room temperature (25° C.) can be reduced to a value lower than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferably lower than or equal to 10 zA, more preferably lower than or equal to 1 zA, still more preferably lower than or equal to 100 yA (1 yA (yoctoampere) is 1×10⁻²⁴ A). In this manner, by using an oxide semiconductor which is made to be an i-type (intrinsic) oxide semiconductor or a substantially i-type oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

Further, in the transistor 162 illustrated in FIGS. 1A and 1B, the cross-sectional shape of the oxide semiconductor layer 144 in the channel-length direction (the direction of carrier flow) is a shape curved along the cross-sectional shape of the trench 131. With this structure, as the trench 131 becomes deeper, an effective channel length of the transistor 162 can be increased.

Therefore, even when the distance between the source electrode 142 a and the drain electrode 142 b is shortened, the effective channel length can be kept by appropriately setting the depth of the trench 131; thus, the occurrence of the short-channel effect can be suppressed and reduction of the area of the transistor can be accomplished. Note that it is preferable that an upper surface shape of the trench 131 be a stripe shape which extends in the channel-width direction of the transistor 162 (the direction perpendicular to the direction of carrier flow).

An example of a manufacturing process of the transistor 162 is described below with reference to FIGS. 2A to 2D.

First, the insulating layer 130 is formed over a substrate (not illustrated) containing a semiconductor material.

For the substrate containing a semiconductor material, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used. A semiconductor element may be formed over the substrate. Note that in general, the term “SOI substrate” means a substrate where a silicon semiconductor layer is provided on an insulating surface. In this specification and the like, the term “SOI substrate” also includes a substrate where a semiconductor layer formed using a material other than silicon is provided over an insulating surface in its category. That is, a semiconductor layer included in the “SOI substrate” is not limited to a silicon semiconductor layer. Moreover, the SOI substrate can be a substrate having a structure in which a semiconductor layer is provided over an insulating substrate such as a glass substrate, with an insulating layer interposed therebetween.

The insulating layer 130 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon oxynitride film, an aluminum oxynitride film, or the like.

Next, the trench 131 (also referred to as a groove) is formed in the insulating layer 130 (see FIG. 2A). The trench 131 can be formed, for example, in such a manner that a resist mask is formed over the insulating layer 130 by a photolithography process and the insulating layer 130 is etched using the resist mask.

For the etching of the insulating layer 130, for example, a dry etching method such as a reactive ion etching (RIE) method, an inductively coupled plasma (ICP) etching method, an electron cyclotron resonance (ECR) etching method, a parallel-plate (capacitively coupled) etching method, a magnetron plasma etching method, a dual-frequency plasma etching method, or a helicon wave plasma etching method can be used. A fluorocarbon-based gas such as trifluoromethane (CHF₃), tetrafluoromethane (CF₄), or perfluorocyclobutane (C₄F₈), methane (CH₄), hydrogen, and a rare gas such as helium or argon can be appropriately mixed to be used as an etching gas.

In addition, the trench 131 is formed in a single etching step or through a plurality of etching steps. In the case of a plurality of etching steps, dry etching and wet etching may be used in combination.

Here, the aspect ratio of the trench 131 is preferably greater than or equal to 0.5 and less than or equal to 5.0. Note that in this specification and the like, the aspect ratio of a trench means a value obtained in such a manner that in a cross section of the trench, the height of an inner wall surface (depth d of the trench 131 in FIG. 1B) is divided by the width of a bottom surface (length L of the trench 131 in FIG. 1B).

When the aspect ratio of the trench 131 is within the above range, film formation is hardly performed over the inner wall surface and the bottom surface of the trench 131 as compared to the uppermost surface of the insulating layer 130; therefore, the oxide semiconductor layer which is formed over the insulating layer 130 in a later step can have different thicknesses without etching using a photomask or the like. In addition, when the aspect ratio of the trench is large, a transistor which is integrated and has less occurrence of a short-channel effect can be obtained.

Then, the oxide semiconductor layer 144 is formed so as to be in contact with the bottom surface and the inner wall surface of the trench 131 provided in the insulating layer 130 (see FIG. 2B). Here, the thickness of the oxide semiconductor layer 144 in contact with the uppermost surface of the insulating layer 130 is preferably greater than or equal to 1 nm and less than or equal to 100 nm In addition, when the aspect ratio of the trench 131 is greater than or equal to 0.5 and less than or equal to 5.0 as described above, the oxide semiconductor layer 144 can be formed in which the thickness of the region in contact with the inner wall surface or the bottom surface of the trench 131 is one-tenth to two-thirds, preferably one-fifth to half, of the thickness of the region in contact with the uppermost surface of the insulating layer 130. Note that in the oxide semiconductor layer 144, the thickness of the region in contact with the inner wall surface of the trench 131 and the thickness of the region in contact with the bottom surface of the trench 131 are not necessarily the same; for example, the thickness of the region in contact with the inner wall surface of the trench 131 may be smaller than the thickness of the region in contact with the bottom surface of the trench 131.

The oxide semiconductor layer 144 can be formed by a sputtering method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition method, an atomic layer deposition (ALD) method, or the like as appropriate.

As a material of the oxide semiconductor layer 144, an oxide semiconductor having a larger band gap than at least silicon is used. As an oxide semiconductor having a larger band gap than silicon, for example, a quaternary metal oxide such as an In—Sn—Ga—Zn—O-based oxide semiconductor; a ternary metal oxide such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxide semiconductor, or a Hf—In—Zn—O-based oxide semiconductor; a binary metal oxide such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxide semiconductor; or a single metal oxide such as an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor can be used. In this embodiment, an In—Ga—Zn—O-based oxide semiconductor is used.

Note that for example, an In—Ga—Zn—O-based oxide semiconductor means an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn), and there is no limitation on the composition ratio thereof

As the oxide semiconductor layer 144, a thin film expressed by a chemical formula of InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metal elements selected from Zn, Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the case where an In—Sn—Zn—O-based oxide semiconductor is used as an oxide semiconductor, a ratio of atoms of the metal elements of the target is In:Sn:Zn=1:2:2, In:Sn:Zn=2:1:3, or In:Sn:Zn=1:1:1 in an atomic ratio, for example.

In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), more preferably, In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). In a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

The deposition atmosphere may be a rare gas (typically argon) atmosphere, an oxygen atmosphere, a mixed atmosphere of a rare gas and oxygen, or the like. An atmosphere of a high-purity gas from which impurities such as hydrogen, water, hydroxyl, and hydride are sufficiently removed is preferable, in order to prevent hydrogen, water, hydroxyl, hydride, and the like from entering the oxide semiconductor layer 144.

The oxide semiconductor layer is in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like.

The oxide semiconductor layer is preferably a CAAC-OS (c-axis aligned crystalline oxide semiconductor) film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor layer with a crystal-amorphous mixed phase structure where crystal parts and amorphous parts are included. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to an a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of an a-axis and a b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor layer, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor layer is higher than that in the vicinity of the surface where the oxide semiconductor layer is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of the c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by deposition or by performing treatment for crystallization such as heat treatment after deposition.

With the use of the CAAC-OS film in a transistor, variation in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability.

When the oxide semiconductor layer 144 is a CAAC-OS film, the oxide semiconductor layer 144 may be formed while the substrate is heated. The temperature of heating the substrate is preferably higher than or equal to 150° C. and lower than or equal to 450° C., and the substrate temperature is preferably higher than or equal to 200° C. and lower than or equal to 350° C. When the substrate is heated at a high temperature during formation of the oxide semiconductor layer, a CAAC-OS film in which the proportion of a crystalline portion is higher than the proportion of an amorphous portion can be formed.

After the formation of the oxide semiconductor layer 144, heat treatment (first heat treatment) may be performed on the oxide semiconductor layer 144. The heat treatment can further remove hydrogen atoms or substances including hydrogen atoms in the oxide semiconductor layer 144. The heat treatment is performed in an inert gas atmosphere at higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. or lower than a strain point of the substrate. The inert gas atmosphere is preferably an atmosphere which contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is higher than or equal to 6N (99.9999%), preferably higher than or equal to 7N (99.99999%) (that is, the concentration of the impurities is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

Note that the heat treatment can be referred to as dehydration treatment, dehydrogenation treatment, or the like because of its advantageous effect of removing hydrogen, water, or the like. The heat treatment can be performed at the timing, for example, after the oxide semiconductor layer is processed into an island shape. Such dehydration treatment or dehydrogenation treatment may be conducted once or plural times.

Next, a conductive layer is formed over the oxide semiconductor layer 144 and is processed, so that the source electrode 142 a and the drain electrode 142 b electrically connected to the oxide semiconductor layer 144 are formed.

The source electrode 142 a and the drain electrode 142 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium, or an alloy material which includes any of these materials as its main component.

Then, the gate insulating layer 146 is formed over the oxide semiconductor layer 144, the source electrode 142 a, and the drain electrode 142 b (see FIG. 2C).

The gate insulating layer 146 can have a thickness of 1 nm to 100 nm and can be formed by a sputtering method, an MBE method, a CVD method, a pulsed laser deposition method, an ALD method, or the like as appropriate. Note that in consideration of contact with the oxide semiconductor layer 144, it is preferable that impurities such as hydrogen be sufficiently removed. Accordingly, it is preferable that the gate insulating layer 146 be formed by a sputtering method by which impurities such as hydrogen are less likely to enter the gate insulating layer 146.

The gate insulating layer 146 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, a silicon nitride oxide film, or the like. Further, the gate insulating layer 146 is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating layer 146.

In this embodiment, a silicon oxide film of SiO_(2/α) (α>0) is used as the gate insulating layer 146. By using the silicon oxide film as the gate insulating layer 146, oxygen can be supplied to the In—Ga—Zn—O-based oxide semiconductor and favorable characteristics can be obtained.

When the gate insulating layer 146 is formed using, for example, a high-k material such as hafnium oxide, yttrium oxide, lanthanum oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added, or hafnium aluminate to which nitrogen is added, gate leakage current can be reduced. Further, the gate insulating layer 146 may have a single-layer structure or a stacked-layer structure.

After the formation of the gate insulating layer 146, second heat treatment may be performed in an inert gas atmosphere or an oxygen atmosphere. The heat treatment temperature is preferably higher than or equal to 200° C. and lower than or equal to 450° C., more preferably higher than or equal to 250° C. and lower than or equal to 350° C. The second heat treatment can reduce variation in electric characteristics of the transistor. Further, in the case where the gate insulating layer 146 in contact with the oxide semiconductor layer 144 contains oxygen, oxygen is supplied to the oxide semiconductor layer 144 to fill oxygen deficiency in the oxide semiconductor layer 144, so that an i-type (intrinsic) or substantially i-type oxide semiconductor layer can be formed.

Note that the second heat treatment is performed after the formation of the gate insulating layer 146 in this embodiment; the timing of the second heat treatment is not limited thereto. For example, the second heat treatment may be performed after the formation of the source electrode 142 a and the drain electrode 142 b. The second heat treatment may be performed following the first heat treatment.

Next, the gate electrode 148 is formed over the oxide semiconductor layer 144 with the gate insulating layer 146 interposed therebetween (see FIG. 2D). In this embodiment, the gate electrode 148 is provided, which fills the trench 131.

The gate electrode 148 can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. As the gate electrode 148, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used. The gate electrode 148 may have a single-layer structure or a stacked-layer structure.

As one layer of the gate electrode 148 which is in contact with the gate insulating layer 146, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (InN, SnN, or the like) film is preferably used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage which is one of the electric characteristics of the transistor to be positive when used as the gate electrode. Accordingly, a so-called normally off switching element can be provided.

In this manner, the transistor 162 in this embodiment can be manufactured. In the transistor 162 in this embodiment, the oxide semiconductor layer 144 is provided in contact with the insulating layer 130 including the trench 131 with an aspect ratio of greater than or equal to 0.5 and less than or equal to 5.0, whereby the oxide semiconductor layer 144 with different thicknesses in regions can be obtained without additional treatment, e.g., etching. In the oxide semiconductor layer 144, a region serving as a source region or a drain region has a thickness larger than a region serving as a channel formation region. Therefore, in the transistor 162 including the oxide semiconductor layer 144, variation in threshold voltage, degradation of electric characteristics, and shift to normally on can be suppressed and source resistance or drain resistance can be reduced, so that a highly reliable transistor can be obtained.

In the transistor 162, the oxide semiconductor layer 144 is provided in contact with the bottom surface and the inner wall surface of the trench 131, whereby the effective channel length of the transistor 162 can be longer than the distance between the source electrode 142 a and the drain electrode 142 b (an apparent channel length of the transistor 162). Accordingly, the occurrence of the short-channel effect can be suppressed and reduction of the area of the transistor can be achieved.

Further, a wide-gap semiconductor having a wide band gap is used in a channel formation region of the transistor 162, whereby the off-state current of the transistor 162 can be reduced. In this embodiment, the highly purified and intrinsic oxide semiconductor layer 144 is used in a channel formation region, whereby the off-state current of the transistor 162 can be further reduced.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

(Embodiment 2)

In this embodiment, an example of a semiconductor device which includes the transistor 162 described in Embodiment 1, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, will be described with reference to the drawings.

Since the off-state current of the transistor 162 is small, stored data can be held for a long time owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor memory device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided.

FIGS. 3A to 3C illustrate one example of a structure of the semiconductor device. FIGS. 3A to 3C illustrate a cross-sectional view, a plan view, and a circuit diagram, respectively, of the semiconductor device. Here, FIG. 3A corresponds to cross sections taken along line C1-C2 and line D1-D2 in FIG. 3B.

The semiconductor device illustrated in FIGS. 3A and 3B includes a transistor 160 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. Note that the structure of the transistor 162 is the same as that described in Embodiment 1; thus, for description of FIGS. 3A and 3B, the same reference numerals are used for the same parts as those in FIGS. 1A and 1B.

Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold charge for a long time owing to its characteristics.

Although all the transistors are n-channel transistors here, it is needless to say that p-channel transistors can be used. The technical nature of the invention disclosed in this embodiment is to use a wide-gap semiconductor in the transistor 162 so that data can be held. Therefore, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to those described here.

The transistor 160 in FIG. 3A includes a channel formation region 116 provided in a substrate 100 including a semiconductor material (e.g., silicon), impurity regions 120 provided such that the channel formation region 116 is sandwiched therebetween, metal compound regions 124 (regions of which resistance is reduced by reaction of a semiconductor material and a metal material) in contact with the impurity regions 120, a gate insulating layer 108 provided over the channel formation region 116, and a gate electrode 110 provided over the gate insulating layer 108.

An electrode 126 is connected to part of the metal compound region 124 of the transistor 160. Here, the electrode 126 functions as a source electrode or a drain electrode of the transistor 160. Further, an element isolation insulating layer 106 is formed over the substrate 100 so as to surround the transistor 160, and the insulating layer 130 is formed to cover the transistor 160. Note that in order to realize high integration, the transistor 160 preferably has a structure without a sidewall insulating layer as illustrated in FIG. 3A. On the other hand, when the characteristics of the transistor 160 have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode 110 and the impurity regions 120 may include a region having a different impurity concentration.

As illustrated in FIG. 3A, the transistor 162 includes a wide-gap semiconductor and has a trench structure. In this embodiment, as the wide-gap semiconductor, the oxide semiconductor layer 144 is used. Here, the oxide semiconductor layer 144 is preferably highly purified. By using a highly purified oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

A conductive layer 153 is provided in a region overlapping with the source electrode 142 a of the transistor 162 with the gate insulating layer 146 interposed therebetween; a capacitor 164 includes the source electrode 142 a, the gate insulating layer 146, and the conductive layer 153. That is, the source electrode 142 a of the transistor 162 functions as one electrode of the capacitor 164, and the conductive layer 153 functions as the other electrode of the capacitor 164. The conductive layer 153 can be manufactured through the same steps as the gate electrode 148.

Note that the capacitor 164 may be omitted if a capacitor is not needed. Further, the capacitor 164 may be separately provided above the transistor 162. For example, a trench-type capacitor or a stack-type capacitor may be separately formed above the transistor 162 or below the transistor 160 so as to be three-dimensionally stacked, whereby the degree of integration may be further increased.

An insulating layer 150 is provided over the transistor 162 and the capacitor 164. Further, a wiring 156 for connecting the transistor 162 to another transistor is provided over the insulating layer 150. The wiring 156 is electrically connected to the drain electrode 142 b through the electrode 154 formed in an opening which is formed in the insulating layer 150, the gate insulating layer 146, and the like. Here, the electrode 154 is preferably provided so as to overlap with at least part of the oxide semiconductor layer 144 of the transistor 162.

In FIGS. 3A and 3B, the transistor 160 is provided so as to overlap with at least part of the transistor 162. The source region or the drain region of the transistor 160 and the oxide semiconductor layer 144 are preferably provided so as to overlap with each other at least partly. Further, the transistor 162 and the capacitor 164 are provided so as to overlap with at least part of the transistor 160. For example, the conductive layer 153 of the capacitor 164 is provided so as to overlap with at least part of the gate electrode 110 of the transistor 160. With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, high integration can be achieved.

Note that although the metal compound region 124, the drain electrode 142 b, and the wiring 156 are connected to one another through the electrode 126 and the electrode 154 in FIG. 3A, the disclosed invention is not limited thereto. For example, the drain electrode 142 b may be in direct contact with the metal compound region 124. Alternatively, the wiring 156 may be in direct contact with the drain electrode 142 b.

Next, an example of a circuit configuration corresponding to FIGS. 3A and 3B is illustrated in FIG. 3C.

In FIG. 3C, a first wiring (a 1st Line) is electrically connected to a source electrode of the transistor 160. A second wiring (a 2nd Line) is electrically connected to a drain electrode of the transistor 160. A third wiring (a 3rd Line) is electrically connected to one of the source electrode and the drain electrode of the transistor 162, and a fourth wiring (a 4th Line) is electrically connected to a gate electrode of the transistor 162. The gate electrode of the transistor 160 and the other of the source electrode and the drain electrode of the transistor 162 are electrically connected to one electrode of the capacitor 164, and a fifth wiring (a 5th line) is electrically connected to the other electrode of the capacitor 164.

The semiconductor device in FIG. 3C utilizes a characteristic in which the potential of the gate electrode of the transistor 160 can be held, and can thus write, hold, and read data as follows.

Writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode of the transistor 160 and the capacitor 164. That is, predetermined charge is given to the gate electrode of the transistor 160 (writing of data). Here, one of two kinds of charges providing different potentials (hereinafter referred to as a low-level charge and a high-level charge) is applied. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge given to the gate electrode of the transistor 160 is held (holding of data).

Since the off-state current of the transistor 162 is extremely small, the charge of the gate electrode of the transistor 160 is held for a long time.

Next, reading of data will be described. By supplying an appropriate potential (reading potential) to the fifth wiring while a predetermined potential (constant potential) is supplied to the first wiring, the potential of the second wiring varies depending on the amount of charge held in the gate electrode of the transistor 160. This is because in general, when the transistor 160 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where a high-level charge is given to the gate electrode of the transistor 160 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where a low-level charge is given to the gate electrode of the transistor 160. Here, an apparent threshold voltage refers to the potential of the fifth wiring, which is needed to turn on the transistor 160. Thus, the potential of the fifth wiring is set to a potential V₀ which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge given to the gate electrode of the transistor 160 can be determined. For example, in the case where a high-level charge is given in writing, when the potential of the fifth wiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 160 is turned on. In the case where a low-level charge is given in writing, even when the potential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), the transistor 160 remains in an off state. Therefore, the data held can be read by measuring the potential of the second wiring.

Note that in the case where memory cells are arrayed to be used, data of only desired memory cells needs to be read. In the case where such reading is not performed, a potential at which the transistor 160 is turned off regardless of the state of the gate electrode of the transistor 160, that is, a potential smaller than V_(th) _(—) _(H) may be given to the fifth wiring. Alternatively, a potential at which the transistor 160 is turned on regardless of the state of the gate electrode, that is, a potential higher than V_(th) _(—) _(L) may be given to the fifth wiring.

In the semiconductor device described in this embodiment, the transistor in which a wide-gap semiconductor (for example, an oxide semiconductor) is used for a channel formation region and in which the off-state current is extremely small is applied, whereby stored data can be held for an extremely long time. In other words, power consumption can be sufficiently reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be sufficiently low. Moreover, stored data can be held for a long time even when power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not occur at all. In other words, the semiconductor device according to one embodiment of the disclosed invention does not have a limit on the number of times of writing which is a problem in a conventional non-volatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily realized.

In addition, by employing the trench structure for the transistor 162, a reduction of the planar area of the transistor 162 can be achieved, so that high integration can be achieved. Since the transistor 162 is highly reliable, a semiconductor device including the transistor can have high reliability.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

(Embodiment 3)

In this embodiment, a semiconductor device which includes the transistor described in Embodiment 1, which can hold stored data even when not powered, which does not have a limitation on the number of write cycles, and which has a structure different from the structure described in Embodiment 2, will be described with reference to FIGS. 4A and 4B and FIGS. 5A and 5B.

FIG. 4A illustrates an example of a circuit configuration of a semiconductor device, and FIG. 4B is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in FIG. 4A will be described, and then, the semiconductor device illustrated in FIG. 4B will be described.

In the semiconductor device illustrated in FIG. 4A, a bit line BL is electrically connected to a source electrode or a drain electrode of the transistor 262, a word line WL is electrically connected to a gate electrode of the transistor 262, and a source electrode or a drain electrode of the transistor 262 is electrically connected to a first terminal of a capacitor 254.

In this embodiment, the transistor including a wide-gap semiconductor, which is described in Embodiment 1, is used as the transistor 262. The transistor 262 including a wide-gap semiconductor has small off-state current. In particular, when a highly purified oxide semiconductor is used as the wide-gap semiconductor, the off-state current can be extremely small. For that reason, a potential of the first terminal of the capacitor 254 (or a charge accumulated in the capacitor 254) can be held for an extremely long time by turning off the transistor 262. Further, in the transistor 262, a short-channel effect is not likely to be caused, which is advantageous.

Next, writing and holding of data in the semiconductor device (a memory cell 250) illustrated in FIG. 4A will be described.

First, the potential of the word line WL is set to a potential at which the transistor 262 is turned on, so that the transistor 262 is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor 254 (writing of data). After that, the potential of the word line WL is set to a potential at which the transistor 262 is turned off, so that the transistor 262 is turned off. Thus, the potential of the first terminal of the capacitor 254 is held (holding of data).

Because the off-state current of the transistor 262 is extremely small, the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor) can be held for a long time.

Secondly, reading of data will be described. When the transistor 262 is turned on, the bit line BL which is in a floating state and the capacitor 254 are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor 254. As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor 254).

For example, the potential of the bit line BL after charge redistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potential of the first terminal of the capacitor 254, C is the capacitance of the capacitor 254, C_(B) is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V_(B0) is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell 250 is in either of two states in which the potentials of the first terminal of the capacitor 254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case of holding the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of the bit line BL in the case of holding the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read.

As described above, the semiconductor device illustrated in FIG. 4A can hold charge that is accumulated in the capacitor 254 for a long time because the off-state current of the transistor 262 is extremely small. In other words, power consumption can be sufficiently reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long time even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 4B will be described.

The semiconductor device illustrated in FIG. 4B includes memory cell arrays 251 a and 251 b including a plurality of memory cells 250 illustrated in FIG. 4A as a memory circuit in an upper portion, and a peripheral circuit 253 in a lower portion which is necessary for operation of the memory cell array 251 (the memory cell arrays 251 a and 251 b ). Note that the peripheral circuit 253 is electrically connected to the memory cell array 251.

In the structure illustrated in FIG. 4B, the peripheral circuit 253 can be provided under the memory cell array 251 (the memory cell arrays 251 a and 251 b ). Thus, the size of the semiconductor device can be decreased.

It is preferable that a semiconductor material of a transistor provided in the peripheral circuit 253 be different from that of the transistor 262. For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed can be favorably realized by the transistor.

Note that FIG. 4B illustrates, as an example, the semiconductor device in which two memory cell arrays 251 (the memory cell arrays 251 a and 251 b ) are stacked; however, the number of memory cells to be stacked is not limited thereto. Three or more memory cells may be stacked.

Next, a specific structure of the memory cell 250 illustrated in FIG. 4A will be described with reference to FIGS. 5A and 5B.

FIGS. 5A and 5B illustrate an example of a structure of the memory cell 250. FIG. 5A illustrates cross-sections of the memory cell 250, and FIG. 5B is a plan view of the memory cell 250. Here, FIG. 5A illustrates cross sections taken along line F1-F2 and line G1-G2 in FIG. 5B.

The structure of the transistor 262 illustrated in FIGS. 5A and 5B is the same as that described in Embodiment 1; thus, for description of FIGS. 5A and 5B, the same reference numerals are used for the same parts as those in FIGS. 1A and 1B.

A conductive layer 264 is provided in a region overlapping with the source electrode 142 a of the transistor 262 with the gate insulating layer 146 interposed therebetween; a capacitor 254 includes the source electrode 142 a, the gate insulating layer 146, and the conductive layer 264. That is, the source electrode 142 a of the transistor 262 functions as one electrode of the capacitor 254, and the conductive layer 264 functions as the other electrode of the capacitor 254.

An insulating layer 258 is provided over the transistor 262 and the capacitor 254. Further, a wiring 260 for connecting the memory cell 250 to an adjacent memory cell 250 is provided over the insulating layer 258. The wiring 260 is electrically connected to the drain electrode 142 b of the transistor 262 through an opening which is provided in the gate insulating layer 146, the insulating layer 258, and the like. Alternatively, the wiring 260 may be electrically connected to the drain electrode 142 b through another conductive layer provided in the opening. Note that the wiring 260 corresponds to the bit line BL in the circuit diagram of FIG. 4A.

In FIGS. 5A and 5B, the drain electrode 142 b of the transistor 262 also functions as a source electrode of the transistor included in the adjacent memory cell. With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, high integration can be achieved.

As described above, the plurality of memory cells formed in multiple layers in the upper portion is each formed with a transistor including an oxide semiconductor as a wide-gap semiconductor layer. Since the off-state current of the transistor including an oxide semiconductor as a wide-gap semiconductor layer is small, stored data can be held for a long time owing to such a transistor. In other words, power consumption can be sufficiently reduced because the frequency of refresh operation can be extremely low.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

(Embodiment 4)

In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or e-book readers will be described with reference to FIGS. 6A and 6B, FIG. 7, FIG. 8, and FIG. 9.

In a portable devices such as a cellular phone, a smartphone, or an e-book reader, an SRAM or a DRAM is used so as to store image data temporarily. The reason why an SRAM or a DRAM is used is that a flash memory is slow in responding and is not suitable for image processing. However, an SRAM or a DRAM has the following features when used for temporary storage of image data.

In a normal SRAM, as illustrated in FIG. 6A, one memory cell includes six transistors, which are a transistor 801, a transistor 802, a transistor 803, a transistor 804, a transistor 805, and a transistor 806, and they are driven by an X decoder 807 and a Y decoder 808. The transistors 803 and 805 and the transistors 804 and 806 each serve as an inverter, and high-speed driving can be performed therewith. However, an SRAM has a disadvantage of large cell area because one memory cell includes six transistors. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F² to 150 F². Therefore, a price per bit of an SRAM is the most expensive among a variety of memory devices.

On the other hand, as illustrated in FIG. 6B, a memory cell in a DRAM includes a transistor 811 and a storage capacitor 812, and is driven by an X decoder 813 and a Y decoder 814. One cell includes one transistor and one capacitor and thus the area of a memory cell is small. The area of a memory cell of a DRAM is generally less than or equal to 10 F². Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed.

However, the area of the memory cell of the semiconductor device described in any of the above embodiments is about 10 F² and frequent refresh operations are not needed. Therefore, the area of the memory cell is reduced, and the power consumption can be reduced.

Next, a block diagram of a portable device is illustrated in FIG. 7. A portable device illustrated in FIG. 7 includes an RF circuit 901, an analog baseband circuit 902, a digital baseband circuit 903, a battery 904, a power supply circuit 905, an application processor 906, a flash memory 910, a display controller 911, a memory circuit 912, a display 913, a touch sensor 919, an audio circuit 917, a keyboard 918, and the like. The display 913 includes a display portion 914, a source driver 915, and a gate driver 916. The application processor 906 includes a CPU 907, a DSP 908, and an interface 909 (IF 909). In general, the memory circuit 912 includes an SRAM or a DRAM; by employing the semiconductor device described in any of the above embodiments for the memory circuit 912, writing and reading of data can be performed at high speed, data can be held for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 8 shows an example in which the semiconductor device described in any of the above embodiments is used for a memory circuit 950 in a display. The memory circuit 950 illustrated in FIG. 8 includes a memory 952, a memory 953, a switch 954, a switch 955, and a memory controller 951. Further, the memory circuit 950 is connected to a display controller 956 which reads and controls image data input through a signal line (input image data) and data stored in the memories 952 and 953 (stored image data), and is also connected to a display 957 which displays an image based on a signal input from the display controller 956.

First, image data (input image data A) is formed by an application processor (not illustrated). The input image data A is stored in the memory 952 through the switch 954. Then, the image data stored in the memory 952 (stored image data A) is transmitted to the display 957 through the switch 955 and the display controller 956, and is displayed on the display 957.

In the case where the input image data A is not changed, the stored image data A is read from the memory 952 through the switch 955 by the display controller 956 normally at a frequency of approximately 30 Hz to 60 Hz.

Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory 953 through the switch 954. The stored image data A is read periodically from the memory 952 through the switch 955 even during that time. After the completion of storing the new image data (the stored image data B) in the memory 953, from the next frame for the display 957, the stored image data B starts to be read, transmitted to the display 957 through the switch 955 and the display controller 956, and displayed on the display 957. This reading operation is continued until another new image data is stored in the memory 952.

By alternately writing and reading image data to and from the memory 952 and the memory 953 as described above, images are displayed on the display 957. Note that the memory 952 and the memory 953 are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory 952 and the memory 953, data can be written and read at high speed and stored for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 9 is a block diagram of an e-book reader. FIG. 9 includes a battery 1001, a power supply circuit 1002, a microprocessor 1003, a flash memory 1004, an audio circuit 1005, a keyboard 1006, a memory circuit 1007, a touch panel 1008, a display 1009, and a display controller 1010.

Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit 1007 in FIG. 9. The memory circuit 1007 has a function of temporarily storing the contents of a book. For example, a user may use a highlight function. In some cases, a user wants to mark a specific portion while reading an e-book reader. This marking function is called a highlight function and is used to make a difference from the other portions by changing the display color, underlining, making characters bold, changing the font of characters, or the like. The function makes it possible to store and hold data of a portion specified by a user. In order to store the data for a long time, the data may be copied to the flash memory 1004. Also in such a case, the semiconductor device described in any of the above embodiments is used, whereby writing and reading of data can be performed at high speed, data can be stored for a long time, and power consumption can be sufficiently reduced.

As described above, the portable devices described in this embodiment each incorporates the semiconductor device according to any of the above embodiments. Therefore, it is possible to obtain a portable device which is capable of reading data at high speed, storing data for a long time, and reducing power consumption.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

EXAMPLE

In this example, an example is described in which an oxide semiconductor layer is formed in contact with an insulating layer having a trench by the manufacturing method described in Embodiment 1.

A method for manufacturing a sample used in this example is described below.

A silicon substrate was used as a substrate, and a silicon oxide film with a thickness of 500 nm was formed over the silicon substrate as an insulating layer by a sputtering method.

The silicon oxide film was formed under the following conditions: a silicon oxide (SiO₂) target was used as a target, the distance between the silicon substrate and the target was 60 mm, the pressure was 0.4 Pa, the radio frequency (RF) power was 2 kW, the atmosphere was a mixed atmosphere of argon and oxygen (the argon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), and the substrate temperature was 100° C.

Next, a resist mask was formed over the silicon oxide film by a photolithography process, and the silicon oxide film was etched using the resist mask to form a trench. The etching step was performed using an inductively coupled plasma (ICP) etching method under the following conditions: trifluoromethane (CHF₃), helium (He), and methane (CH₄) (CHF₃:He:CH₄=22.5 sccm:127.5 sccm: 5 sccm) were used as an etching gas, the power of the power source was 475 W, the bias power was 300 W, the pressure was 3.5 Pa, and the etching time was 96 seconds. After the etching step, ashing with oxygen (the power of the power source was 200 W, the pressure was 67 Pa (0.5 Torr), and the time was 300 seconds) was carried out. The length of the inner wall surface of the trench in the cross section (the depth d of the trench in FIG. 1B) was 310 nm, and the width of the bottom surface of the trench in the cross section (the length L of the trench in FIG. 1B) was 330 nm. In other words, the aspect ratio of the manufactured trench was 0.94.

The resist mask over the silicon oxide film was removed using a stripper, and an In—Ga—Zn—O film was formed as an oxide semiconductor layer in contact with the inner wall surface of the trench, the bottom surface of the trench, and the uppermost surface of the insulating layer by a sputtering method.

In this example, the oxide semiconductor layer was formed while the substrate was heated to 400° C. The In—Ga—Zn—O film was formed under the following conditions: an oxide target having a composition ratio of In:Ga:Zn=1:1:1[atomic ratio] was used, the distance between the silicon substrate and the target was 60 mm, the pressure was 0.4 Pa, the direct current (DC) power was 0.5 kW, and the atmosphere was a mixed atmosphere of argon and oxygen (the argon flow rate was 30 sccm and the oxygen flow rate was 15 sccm). The oxide semiconductor layer was formed to have a thickness of 40 nm It is preferable that argon and oxygen used for formation of the oxide semiconductor layer do not contain water, hydrogen, and the like. For example, it is preferable that argon have a purity of 9N, a dew point of −121° C., a water content of 0.1 ppb, and a hydrogen content of 0.5 ppb, and oxygen have a purity of 8N, a dew point of −112° C., a water content of 1 ppb, and a hydrogen content of 1 ppb.

FIG. 10 is a cross-sectional transmission electron microscopy (TEM) photograph of the sample of this example obtained through the above steps.

FIG. 10 indicates that an oxide semiconductor layer 320 provided over the insulating layer 310 including the trench has different thicknesses in regions; the thickness of a region 320 b in contact with the inner wall surface of the trench and the thickness of a region 320 a in contact with the bottom surface of the trench are each smaller than the thickness of a region 320 c in contact with the uppermost surface of the insulating layer 310. In the oxide semiconductor layer 320, the thickness of the region 320 b in contact with the inner wall surface of the trench was 16 nm, the thickness of the region 320 a in contact with the bottom surface of the trench was 21 nm, and the thickness of the region 320 c in contact with the uppermost surface of the insulating layer 310 was 40 nm.

As described above, in this example, when the oxide semiconductor layer 320 is provided in contact with the insulating layer 310 having the trench, the oxide semiconductor layer 320 having different thicknesses in regions can be obtained without additional treatment, e.g., etching. When the oxide semiconductor layer 320 is used for a transistor, source resistance or drain resistance can be reduced and variation in threshold voltage, degradation of electric characteristics, shift to normally on can be suppressed, so that the transistor can be highly reliable.

This application is based on Japanese Patent Application serial no. 2011-096163 filed with Japan Patent Office on Apr. 22, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a trench in an insulating layer; a wide-gap semiconductor layer in contact with an uppermost surface of the insulating layer and an inner wall surface and a bottom surface of the trench; a source electrode and a drain electrode electrically connected to the wide-gap semiconductor layer; a gate insulating layer over the wide-gap semiconductor layer; and a gate electrode over the gate insulating layer, wherein each of a first thickness of the wide-gap semiconductor layer in contact with the inner wall surface and a second thickness of the wide-gap semiconductor layer in contact with the bottom surface is smaller than a third thickness of the wide-gap semiconductor layer in contact with the uppermost surface of the insulating layer, and wherein the trench is filled with the gate electrode.
 2. The semiconductor device according to claim 1, wherein an aspect ratio of the trench is greater than or equal to 0.5 and less than or equal to 5.0.
 3. The semiconductor device according to claim 1, wherein the third thickness is greater than or equal to 1.5 times and less than or equal to 10 times as large as each of the first thickness and the second thickness.
 4. The semiconductor device according to claim 1, wherein the wide-gap semiconductor layer is an oxide semiconductor layer.
 5. A semiconductor device comprising: a first transistor; and a second transistor, the second transistor comprising: a trench in an insulating layer; a wide-gap semiconductor layer in contact with an uppermost surface of the insulating layer and an inner wall surface and a bottom surface of the trench; a source electrode and a drain electrode electrically connected to the wide-gap semiconductor layer; a gate insulating layer over the wide-gap semiconductor layer; and a gate electrode over the gate insulating layer, wherein the first transistor overlaps with at least a part of the second transistor, wherein each of a first thickness of the wide-gap semiconductor layer in contact with the inner wall surface and a second thickness of the wide-gap semiconductor layer in contact with the bottom surface is smaller than a third thickness of the wide-gap semiconductor layer in contact with the uppermost surface of the insulating layer, and wherein the trench is filled with the gate electrode.
 6. The semiconductor device according to claim 5, wherein an aspect ratio of the trench is greater than or equal to 0.5 and less than or equal to 5.0.
 7. The semiconductor device according to claim 5, wherein the third thickness is greater than or equal to 1.5 times and less than or equal to 10 times as large as each of the first thickness and the second thickness.
 8. The semiconductor device according to claim 5, wherein an oxide semiconductor layer is used as the wide-gap semiconductor layer. 